Tracking of surgical instruments or tools is an integral part of computer-assisted surgery (hereinafter CAS). The tools are tracked for position and/or orientation in such a way that information pertaining to bodily parts is obtained. The information is then used in various interventions (e.g., orthopedic surgery, neurological surgery) with respect to the body, such as bone alterations, implant positioning, incisions and the like during surgery.
The tracking systems may use different technologies, such as mechanical, acoustical, magnetic, optical and RF tracking. Depending on the technology used, different types of trackable references are fixed, permanently or temporarily, to the item that needs to be tracked. For instance, during Total Knee Replacement (TKR) surgery, trackable references are fixed to the limbs and to the different surgical instruments, and these trackable references are tracked by the tracking system. The CAS system calculates position and orientation data associated with the tracking, and the information displayed by the computer is used by the surgeon to visualize the position of the instrument(s) being manipulated with respect to the limbs, or in numerical values.
Two types of tracking systems are commonly used. The active tracking systems provide a transmitter as trackable reference on the tool to be tracked, which transmitter emits signals to be received by a processor of the CAS system, which will calculate the position and/or orientation of the tool as a function of the signals received. The transmitters of the active tracking systems are powered, for instance by being wired to the CAS system or by being provided with an independent power source, so as to emit signals.
Passive tracking systems do not provide active transmitters on the tools as trackable references. The CAS system associated with passive tracking has an optical sensor apparatus provided to visually detect optical elements on the tools. The optical elements are passive, whereby no power source is associated therewith.
In order to obtain values for position and/or orientation, the optical elements must be in the line of sight of the optical sensor apparatus. Accordingly, with passive tracking systems, surgery takes place in a given orientation as a function of the required visibility between the optical sensor apparatus and the optical elements.
The trackable references currently used, whether active or passive, have a noticeable size depending on the technology used. For an electromagnetic system, a casing is wired to the CAS system and is secured to the instrument or to the patient. For an optical system, a trackable reference generally comprises at least three optical elements in order to provide six degrees of freedom (DOF). For instance, the optical elements are light sources wired to the CAS system and forming a scalene triangle. The light sources can be individually fixed or assembled on a base. In this second construction, the assembly is large and obstructive.
As an alternative, passive reflector spheres or patches can be used instead of light sources, and a light source is used to illuminate them (in the infrared spectrum).
Some factors must be considered when selecting a type of tracking system: the presence of wires in sterile zones for active trackable references; a line of sight required for navigation when using optical tracking; the size of the trackable references in order to deliver the required precision during surgery; the necessity for the surgeon to visualize a computer screen for intraoperative alignment information; the necessity for the surgeon to digitize landmarks on bones in order to build coordinate systems; the difficulty in integrating current optical or radio-frequency sensors in disposable instruments (such as cutting guides) because of their volume. Electromagnetic tracking devices are subject to distortions introduced by conventional orthopaedic instruments which may be difficult to detect and may cause a loss in accuracy. These tracking devices are used as general data input devices, digitizing points on patients or surgical instruments in order to compute planes, point-to-point distances, planar angles, planar distances, etc., required during CAS.
No alternate miniaturized technologies with fewer than 6 DOF is currently used in orthopaedic CAS, while still providing the crucial information required to install orthopaedic implants. Such technology could be directly integrated to instruments, thus reducing the need for an external tracking system, thereby resulting in enhanced ease-of-use.